1. Field of Art
This invention relates to the improvement of a hydro-mechanical transmission. More specifically, the invention relates to the use of fixed displacement hydraulic pump/motors rather the variable displacement pump/motors.
2. Description of the Prior Art
Most if not all hydrostatic and hydro-mechanical transmissions rely on variable-displacement hydraulic pump/motors as a means of changing the transmission ratio of input rotational speed to output rotational speed. Hydro-mechanical transmissions also contain a parallel variable all-mechanical path. The size, cost, complexity, noise, failure rate, and energy losses (under some conditions) of variable displacement pumps and motors are much greater than fixed displacement devices of the same max. displacement and are to the detriment of the transmission. Many transmissions achieve variability of the transmission ratio by varying the amount of power that is re-circulated within the transmission. These re-circulating schemes result in large power losses due to the inefficiency of the power transfer between the elements of the transmission. Many prior art transmissions contain a variable mechanical path in parallel with a variable hydraulic path. Variability is obtained by varying the amount of power transmitted by both these paths. They result in some power being transmitted by the hydraulic path almost all the time. The low efficiency of the hydraulic path causes large losses and fluid heating when operated at high speeds for extended periods.
A hydro-mechanical transmission that would use only fixed-displacement hydraulic devices, contain a highly efficient means of re-circulating power and have a high efficiency mechanical by-pass of the hydraulic portion of the hydraulic transmission, would be an improvement to the state of the art.